1. Field of the Invention
The present invention relates to a transparent conductive film, semiconductor device and active matrix display unit.
2. Description of Related Art
Recent years in the field of display unit using a semiconductor device, flat panel display units including liquid crystal displays and Electro Luminescence display units characterized by energy and space saving are rapidly expanding to take place conventional CRTs. In these display units, several electrodes, wirings and devices are provided on a substrate. Specifically, an active matrix type TFT array substrate including switching devices such as a thin-film transistor (TFT) having a scanning and signal lines, a gate electrode and source/drain electrode are provided in array for applying an independent video signal to electrodes of each display pixel are becoming to be common.
An active matrix type TFT array substrate used in a liquid crystal display is disclosed in Japanese Unexamined Patent Application Publication No. 10-268353. The active matrix type TFT array substrate includes a metal film for forming an electrode and wiring and a transparent conductive layer for forming a pixel electrode and input/output terminal unit of a video signal. ITO and IZO or the like is used for the transparent conductive layer. In general, there are a plurality of connectors provided in which the metal films and the transparent electrode layers are electrically connected.
Further, there is a problem in signal delays caused by longer scanning and signal lines and smaller line width thereof associated with a larger and highly-defined liquid crystal display. To prevent such problem, materials of electrodes and lines are desired to have low resistance electrically such as Al. However, if using Al for metal films, a favorable electric contact characteristic with a transparent electrode layer including ITO and IZO or the like cannot be obtained. Therefore as disclosed in Japanese Unexamined Patent Application Publication No. 2000-77666, generally a method has been used in which a high-melting point metal film such as Ti, Cr and Mo is formed in the connectors between the metal and transparent electrode films to obtain a favorable electric contact characteristic between Al film and the transparent electrode layer through the high-melting point metal film.
Further as a reflecting electrode to display images by reflecting light, there are display units using a pixel electrode made from Al or Al alloy film. For example in a reflective type liquid crystal display, to match with the standard potential of an opposing electrode made from an ITO film, an ITO or IZO film is formed on a pixel electrode acting also as a reflecting electrode that is made from Al or Al alloy film (see Japanese Unexamined Patent Application Publication No. 2004-294804). In an organic EL display unit, to improve the efficiency of injecting charge from an anode pixel electrode to an organic EL light-emitting device, an ITO or IZO film having high work function is formed on a pixel electrode acting also as a reflecting electrode that is made from Al or Al alloy film. In these display units, an ITO or IZO film must be directly formed on an Al or Al alloy film in order to utilize the high optical reflectance of the Al or Al alloy film.
However, to form a film by laminating Al and high-melting point metal film as in Japanese Unexamined Patent Application Publication No. 2000-77666, depending on the kind of the high-melting point metal layer, an edge part of the laminated layer line pattern could be an inverse taper or an appentice shape due to a difference of corrosion potentials in etchant of a etching process for patterning, and thus it has been a problem that a coverage failure is generated in a film formed in a upper layer.
Further, an ITO film directly formed on the Al or Al ally film is polycrystal. As polycrystal ITO is chemically stable, it is patterned using strongly acidic solution of aqua regia system. However, there is a problem of using such strong acidic solution that the underlying Al or Al ally film is etched. On the other hand to prevent this, there is another method to use weak acid solution. In such case, the ITO film is amorphous. An amorphous ITO film is formed by sputtering method in gas mixed with Ar and H2O or H2. However, when mixing H2O or H2, dust is generated in the sputtering and mixed in the ITO film as foreign matter, thus decreasing yield. Furthermore as the amorphous ITO film is crystallized by heating of approx. 150 degree Celsius, it is partially crystallized in a post baking process (generally from 140 to 160 degree Celsius) of a resist patterning for a photolithography process. There is a problem that the ITO crystal grains become etching residue and thereby decreases the yield.
On the other hand, an IZO film directly formed on the Al or Al alloy film is amorphous. As the Amorphous IZO film is formed by a sputtering method not mixing H2O or H2 but only using Ar gas, the abovementioned problem of dust is not created. Furthermore, as the amorphous IZO film is not crystallized even after a post baking process, the above problem of etching residue is not created. However, the IZO film is difficult to crystallize conversely and has poor acid resistance. If an etching by acid chemical or cleansing process is required after forming an IZO film pattern, it may be corroded and deteriorated reliability, thus creating a problem that an incorporation of the IZO film to a device is seriously limited.